Catalytically enhanced reaction processes are used in the synthesis of hydrocarbons and their oxygenated derivatives by the catalytic hydrogenation of carbon monoxide (the Fischer-Tropsch process) and in the epoxidation of alkenes. Such reactions are typically highly exothermic. They are generally performed in a vertical shell-and-tube exchanger type reactor comprising a multitude of reaction tubes, each containing a solid particulate catalyst and surrounded by a heat-exchange fluid.
Typically, a production of ethylene oxide can be performed by the catalytic oxidation in the vapour phase of ethylene with a molecular oxygen-containing gas. Shell-and-tube reactors used for ethylene oxidation contain several thousands of reaction tubes, each 6-15 m long and each having an inside diameter of between 20-50 mm. The ethylene oxidation solid particulate catalysts are generally based on silver supported on an inert carrier material, to which promoters and co-promoters may be added. Supported ethylene oxidation catalysts comprising silver and one or more of the alkali metals K, Rb and Cs are disclosed in U.S. Pat. Nos. 3,962,136 and 4,010,115. Supported ethylene oxidation catalysts comprising silver, rhenium, at least one further metal and optionally a rhenium co-promoter are disclosed in EP-B 0 266 015. A heat-exchange fluid can be a hydrocarbon or a mixture of hydrocarbons, or it can be water.
The reaction of ethylene oxidation is conducted at a temperature which is broadly in the range of from 150 to 350° C. Depending on the reactor design, the catalyst composition, the feed composition and the further reaction conditions a narrow reaction temperature range has to be maintained within any given reactor. The operation involves the pre-heating of the incoming feed gases in the upstream portion of the reaction tubes to the required reaction temperature, removing excess heat during the reaction, and cooling down the effluent of the reaction.
For greater efficiency of the pre-reaction heating and post-reaction cooling, the upstream and/or downstream portions of the ethylene oxidation reactor tubes are traditionally packed with particulate material (also termed packing). Such particulate material may or may not be the same as the particulate catalyst used. In the former case, expensive catalyst is being used as an inert. In U.S. Pat. No. 5,292,904 the use is disclosed of inert particles as packing material in an upstream preheating zone and in a downstream cooling zone of multi-tubular reactors for the production of ethylene oxide. U.S. Pat. No. 4,061,659 is directed to a process for the production of ethylene oxide in a multi-tubular reactor, whereby the downstream portion of the reactor tubes is filled with an inert refractory particulate material having a surface area of 0.1 m2/g or less.
Japanese patent publication JP2000-169463-A envisages the use of stainless steel packing as inert material, the main advantage being that stainless steel is less prone to by-product formation. It should be noted that it is well known, see e.g. Wakao & Kaguei, Heat transfer in packed beds; Gordon & Breach, 1982), that for tubes filled with particulates, the thermal conductivity of the packing material has little influence on the heat transfer under turbulent flow conditions. Therefore, there would be no significant improvement of heat transfer expected when applying (stainless) steel as compared to low conductivity materials such as silica.
When inert particles are used in the upstream and/or downstream portion of the reactor tubes, they have the advantages of being less expensive and having a longer useful life than catalyst particles. On the other hand, their separation from exhausted catalyst introduces a complication.
There exists therefore a need for the use of a different kind of inserts in the upstream and/or downstream portion of the reactor tubes. These inserts should be at least as efficient as the known particulate filling materials in promoting the pre-heating and/or post-cooling of the feed gases. In particular, their heat transfer coefficient should be high and the gas pressure drop caused by them should be low. Moreover, they should be inexpensive and easily separable from the catalyst particles.
The use of coherent inserts of varying and more or less complicated shapes is known in the art of heat exchange, but not in chemical reactors. They are generally designed to cause maximal turbulence of the fluid—mostly liquid—flowing through. Indeed, they are frequently called turbulators or turbulence inserts.
An example of an insert having a complicated shape is to be found in EP-B 0 061 154, wherein an insert for placement in a vessel is disclosed, comprising an elongated core having a plurality of loops disposed longitudinally therealong and angularly thereabout wherein a portion of each loop lies in close proximity to a conceptual enveloping surface, so that laterally rectilinear lines (L) extending from the core to the midpoint of said portion are disposed at different longitudinal positions along the core, characterised in that each line (L) of each loop lies at an acute angle to the core. Preferably, the core comprises two elongated elements twisted together, a part of each of said loops being held between the twisted together elements.
In U.S. Pat. No. 5,454,429 a forced-flow heat exchanger is disclosed, comprising a housing as part of an outer thermal cycle and at least one tube attached in parallel to the longitudinal housing axis as part of an inner thermal cycle, wherein each of said tubes comprises a loosely inserted flexible rod which is freely movable and rotable in the axis and radial tube directions. The flexibility of the rods and their being freely movable are stated to be very essential features, since the resulting vibrations promote a turbulent flow inside the tubes. The rods according to this disclosure are relatively thin, the preferred ratio between the inside tube diameter and the rod diameter being from 1.4 to 2.5. The rods occupy the entire length of the tubes.
This and the further art searched contains no reference to the use of any type of coherent (as opposed to particulate) insert for promoting the pre-heating of reactants in a chemical multi-tubular reactor, in particular when the reactants are gaseous and more in particular when they comprise a carbon monoxide/hydrogen or an ethylene/oxygen reaction mixture.